Magnetic fields and the dynamics of spiral galaxies

We investigate the dynamics of magnetic fields in spiral galaxies by performing 3D magnetohydrodynamics simulations of galactic discs subject to a spiral potential using cold gas, warm gas and a two-phase mixture of both. Recent hydrodynamic simulations have demonstrated the formation of interarm spurs as well as spiral arm molecular clouds, provided the interstellar medium model includes a cold H i phase. We find that the main effect of adding a magnetic field to these calculations is to inhibit the formation of structure in the disc. However, provided a cold phase is included, spurs and spiral arm clumps are still present if β≳ 0.1 in the cold gas. A caveat to the two-phase calculations though is that by assuming a uniform initial distribution, β≳ 10 in the warm gas, emphasizing that models with more consistent initial conditions and thermodynamics are required. Our simulations with only warm gas do not show such structure, irrespective of the magnetic field strength. Furthermore, we find that the introduction of a cold H i phase naturally produces the observed degree of disorder in the magnetic field, which is again absent from simulations using only warm gas. Whilst the global magnetic field follows the large-scale gas flow, the magnetic field also contains a substantial random component that is produced by the velocity dispersion induced in the cold gas during the passage through a spiral shock. Without any cold gas, the magnetic field in the warm phase remains relatively well ordered apart from becoming compressed in the spiral shocks. Our results provide a natural explanation for the observed high proportions of disordered magnetic field in spiral galaxies and we thus predict that the relative strengths of the random and ordered components of the magnetic field observed in spiral galaxies will depend on the dynamics of spiral shocks.

[1]  Spiral shocks and the formation of molecular clouds in a two-phase medium , 2007, astro-ph/0701822.

[2]  Ryoji Matsumoto,et al.  Global Three-dimensional Magentohydrodynamic Simulations of Galactic Gaseous Disks. I. Amplification of Mean Magnetic Fields in an Axisymmetric Gravitational Potential , 2006, astro-ph/0601358.

[3]  William H. Press,et al.  Dynamic mass exchange in doubly degenerate binaries I , 1990 .

[4]  R. Wielebinski Observations of Magnetic Fields in Galaxies , 1992 .

[5]  D. Sokoloff,et al.  Galactic Magnetism: Recent developments and perspectives , 1996 .

[6]  Formation of Spiral-Arm Spurs and Bound Clouds in Vertically Stratified Galactic Gas Disks , 2006, astro-ph/0603751.

[7]  Joseph John Monaghan,et al.  SPH and Riemann Solvers , 1997 .

[8]  Instabilities of spiral shocks – I. Onset of wiggle instability and its mechanism , 2003, astro-ph/0308203.

[9]  A. Lazarian,et al.  Magnetic fields in the universe : from laboratory and stars to primordial structures, Angra dos Reis, Brazil, 28 November - 3 December 2004 , 2005 .

[10]  W. Roberts,et al.  Application of the density-wave theory to the spiral structure of the Milky Way system. III - Magnetic field - Large-scale hydromagnetic shock formation , 1970 .

[11]  Daniel J. Price,et al.  Smoothed Particle Magnetohydrodynamics – I. Algorithm and tests in one dimension , 2003, astro-ph/0310789.

[12]  K. Ferrière Global Model of the Interstellar Medium in our Galaxy with New Constraints on the Hot Gas Component , 1998 .

[13]  Constraining corotation from shocks in tightly wound spiral galaxies , 2003, astro-ph/0312562.

[14]  The Neutral Atomic Phases of the ISM in the Galaxy , 2002, astro-ph/0207098.

[15]  J. Monaghan Smoothed particle hydrodynamics , 2005 .

[16]  E. Ostriker,et al.  Accepted for publication in the Astrophysical Journal Amplification, Saturation, and Q Thresholds for Runaway: Growth of Self-Gravitating Structures in Models of Magnetized Galactic Gas Disks , 2001 .

[17]  R. F. Loewenstein,et al.  Results of SPARO 2003: Mapping Magnetic Fields in Giant Molecular Clouds , 2006 .

[18]  Mpia,et al.  Exploring spiral galaxy potentials with hydrodynamical simulations , 2003, astro-ph/0309597.

[19]  W. Roberts Large-Scale Shock Formation in Spiral Galaxies and its Implications on Star Formation , 1969 .

[20]  D. Balsara,et al.  Amplification of Interstellar Magnetic Fields by Supernova-driven Turbulence , 2004 .

[21]  A. Tielens,et al.  Neutral Atomic Phases of the Interstellar Medium in the Galaxy , 2003 .

[22]  A. Lyne,et al.  Pulsar Rotation Measures and the Large-Scale Structure of the Galactic Magnetic Field , 2006, astro-ph/0601357.

[23]  E. Parker The dynamical state of the interstellar gas and field. II. , 1966 .

[24]  E. Parker The generation of magnetic fields in astrophysical bodies. III - Turbulent diffusion of fields and efficient dynamos , 1971 .

[25]  M. Hanasz,et al.  On the influence of cooling and heating processes on the Parker instability – II. Numerical simulations , 2007 .

[26]  E. Ostriker,et al.  Interstellar Turbulence Driving by Galactic Spiral Shocks , 2006, astro-ph/0608161.

[27]  Daniel J. Price,et al.  An energy‐conserving formalism for adaptive gravitational force softening in smoothed particle hydrodynamics and N‐body codes , 2006, astro-ph/0610872.

[28]  Spurs and feathering in spiral galaxies , 2006, astro-ph/0602100.

[29]  J. Stil,et al.  Cold H I in turbulent eddies and galactic spiral shocks , 2006, Proceedings of the International Astronomical Union.

[30]  Accepted for publication in the Astrophysical Journal Global Modeling of Spur Formation in Spiral Galaxies , 2006 .

[31]  F. Adams,et al.  Star Formation in Molecular Clouds: Observation and Theory , 1987 .

[32]  The Millennium Arecibo 21 Centimeter Absorption-Line Survey. IV. Statistics of Magnetic Field, Column Density, and Turbulence , 2005, astro-ph/0501482.

[33]  Daniel J. Price,et al.  Smoothed particle magnetohydrodynamics - III. Multidimensional tests and the B = 0 constraint , 2005, astro-ph/0509083.

[34]  Donald P. Cox,et al.  THE THREE-PHASE INTERSTELLAR MEDIUM REVISITED , 2005 .

[35]  Three-dimensional Simulations of the Parker Instability in a Uniformly-rotating Disk , 1999, astro-ph/9912300.

[36]  Magnetic Field Structure from Synchrotron Polarization , 2006, astro-ph/0603531.

[37]  C. Heiles,et al.  Magnetic fields in diffuse HI and molecular clouds , 2005, astro-ph/0501550.

[38]  L. Blitz Evolution in the interstellar medium , 2018, Dust in the Galactic Environment.

[39]  D. Sokoloff,et al.  Depolarization and Faraday effects in galaxies , 1998 .

[40]  B. Elmegreen Formation of giant cloud complexes by the Parker-Jeans instability , 1982 .

[41]  E. Ostriker,et al.  Saturated-State Turbulence and Structure from Thermal and Magnetorotational Instability in the ISM: Three-dimensional Numerical Simulations , 2005, astro-ph/0504669.

[42]  Formation and Fragmentation of Gaseous Spurs in Spiral Galaxies , 2001, astro-ph/0111398.

[43]  The formation of molecular clouds in spiral galaxies , 2006, astro-ph/0602103.

[44]  J. E. Pringle,et al.  Spiral shocks, triggering of star formation and the velocity dispersion in giant molecular clouds , 2006 .

[45]  R. Beck,et al.  Cosmic magnetic fields , 2005 .

[46]  C. Heiles,et al.  THE MILLENNIUM ARECIBO 21-CM ABSORPTION LINE SURVEY . II . PROPERTIES OF THE WARM AND COLD NEUTRAL MEDIA , 2002 .

[47]  E. Ostriker,et al.  Accepted for publication in the Astrophysical Journal Three-Dimensional Simulations of Parker, Magneto-Jeans, and Swing Instabilities in Shearing Galactic Gas Disks , 2002 .

[48]  Christopher Fulton McKee The three phase model of the interstellar medium: where does it stand now? , 1990 .

[49]  E. Parker The generation of magnetic fields in astrophysical bodies. II - The galactic field , 1971 .

[50]  F. Shu,et al.  Galactic Shocks in an Interstellar Medium with Two Stable Phases , 1971 .

[51]  Daniel J. Price,et al.  The impact of magnetic fields on single and binary star formation , 2007, astro-ph/0702410.

[52]  Clumpy and fractal shocks, and the generation of a velocity dispersion in molecular clouds , 2006, astro-ph/0610720.

[53]  Branch, Spur, and Feather Formation in Spiral Galaxies , 2003, astro-ph/0306472.

[54]  Eric Ronald Priest,et al.  Solar magneto-hydrodynamics , 1982 .

[55]  P. Woodward Shock-driven implosion of interstellar gas clouds and star formation. , 1976 .

[56]  B. Elmegreen Supercloud formation by nonaxisymmetric gravitational instabilities in sheared magnetic galaxy disks , 1987 .

[57]  Analysis of spiral arms using anisotropic wavelets: gas, dust and magnetic fields in M51 , 2006, astro-ph/0609787.

[58]  R. Beck,et al.  Magnetic spiral arms in the galaxy NGC6946 , 1996, Nature.

[59]  Daniel J. Price,et al.  Smoothed Particle Magnetohydrodynamics – II. Variational principles and variable smoothing-length terms , 2003, astro-ph/0310790.

[60]  J. Vallée Magnetic field versus gas density, in different physical conditions , 1995 .

[61]  R. Laing A model for the magnetic-field structure in extended radio sources , 1980 .

[62]  Woong-Tae Kim,et al.  Magnetorotationally Driven Galactic Turbulence and the Formation of Giant Molecular Clouds , 2003, astro-ph/0309080.

[63]  R. Kulsrud,et al.  The stabilizing effects of cloud reacceleration - Microturbulence, and rotation of Parker's instability , 1975 .